1. Field of Invention
This invention relates generally to a non-contacting, electro-optical measuring system for mapping the contoured surface of an object being tested, such as the contour of a helicopter rotor blade, and more particularly to a system of this type which makes use of an electro-optical triangulation rangefinder assembly to effect accurate and rapid contour measurements.
2. Prior Art
Though the invention is applicable to contour measurement of any three-dimensional surface without making contact with the surface being measured, we shall for purposes of explanation describe the invention in connection with contour measurement of helicopter rotor blade surfaces. Such blades are relatively massive and present particular problems which are solved by the present invention.
The performance of a helicopter and its operating life are greatly affected by the extent to which the rotor blades adhere to predetermined contour design requirements. If the manufactured blade deviates significantly therefrom, the rotating blade is subject to vibration which not only degrades the performance of the vehicle in flight, but also reduces the life of the blade and the gear box associated therewith.
The manufacture of many precision parts such as instrumentation components and various shaped turbine and propeller blades, entails close control of and the ability to determine the contour of various surfaces on these parts. Conventional gauging systems for this purpose make use of contact probes that physically engage the surface under observation to carry out the required contour measurement. To a large degree, the accuracy of these measurements depends upon the stability of the mechanical structure which serves as a reference, for any twisting, bending or settling of the structure that takes place after the measuring gauge is calibrated or in the course of measurement will adversely affect the accuracy of the reading.
When the object being measured is a helicopter blade, contact probes must cover over 40 feet in the span direction and 48 inches in the chord direction with a position accuracy of better than plus or minus 0.001 inches under normal shop conditions. The cost and complexity of a mechanical X-Y carriage for orienting a contour measuring probe with this order of accuracy is exceptionally high. If multiprobes are used, the relative position of one to the other must be known and held to better than plus or minus 0.001 inches. This too is very expensive to realize in practice. Moreover, contact probes are subject to wear and must be replaced at fairly frequent intervals.
Another practical drawback of conventional contact-type contour gauging systems is that they afford little, if any, flexibility in operation. Once the probes are positioned for a certain type of blade, before any other blade configuration can be measured, a costly probe rearrangement is entailed.
To effect contour mapping of shaped objects, it is also known to use non-contact, electro-optical systems such as those described in the Waters U.S. Pat. No. 3,909,131; the Lowrey et al. U.S. Pat. No. 3,986,774; the Erb U.S. Pat. No. 3,671,126 and the Zoot et al. U.S. Pat. No. 3,679,307. These prior patents describe electro-optical techniques for surface gauging and represent noncontacting optical concepts predicated on geometrical relationships between a source of radiation and a detector.
The above-identified Waters patent is of particular interest; for the invention disclosed in this patent is based on the principle that as a focused spot of radiation from a laser source is moved in discrete increments across the surface being gauged, contour variations can be measured by triangulation, this being accomplished by electronically following the position of an image of the spot on a detector array. Two modes of operation are disclosed by Waters. In one mode, the object being tested is translated until the imaged spot is driven through a null position. In the other mode, it is the detector that is translated relative to the object.
In a non-contacting system of the Waters type operating in the mode in which the contoured object is physically displaced with respect to the triangulation system, this translation results in shifting masses which act to flex and distort the system, thereby giving rise to significant measurement errors, particularly when contour mapping large surfaces such as helicopter rotor blades.
Alternately, when Waters translates his detector, this translation generates optical and mechanical errors in rotational angle which are difficult to calibrate or control. These drawbacks are acknowledged by Waters who recommends object translation as "a superior operational mode from an accuracy consideration" (column 7, lines 24 and 25).
Also of interest are the following prior art patents relating to non-contact electro-optical measuring systems: Nordqvist, U.S. Pat. No. 3,655,990; Foster et al., 3,858,983; Zanoni, 3,768,910; Zanoni, 3,847,485 and Zanoni, 3,907,439.